Atmospheric air vaporizers (ambient air vaporizers) are well known in the art and are used in many cryogenic liquid plants to vaporize cryogenic liquids, such as liquid nitrogen for industrial usage. In most cases, ambient air vaporizers are based on a heat exchanger which uses sensible heat of ambient air and/or latent heat of water in the environment to heat a low boiling point liquid (e.g., liquid oxygen, liquid nitrogen, etc.). The vaporization duty of these vaporizers is relatively small when compared to the large duties required by LNG regasification terminals. Therefore, application of known ambient air vaporizers for regasification of LNG requires a rather significant large plot space, which is uneconomical and/or impractical, especially in offshore and floating LNG regasification facilities.
State of the art ambient air vaporizers/heat exchangers typically include a number of individual multi-finned heat transfer elements in various serial and/or parallel configurations. Such finned heat exchangers are relatively efficient for transferring heat from the ambient air to vaporize and superheat LNG due to the large temperature difference between ambient air and LNG. Most of these exchangers are in vertical orientation and have counter current flow between the downward cold denser air (due to gravitational force) and the upward flow of the LNG in the vaporizer tubes. For example, U.S. Pat. Nos. 4,479,359 and 5,252,425 show exemplary configurations for ambient air vaporizers. Further known and similar LNG regasification configurations are described in U.S. Pat. App. No. 2006/0196449, U.S. Pat. No. 7,155,917, and JP 05312300.
In all of such known ambient exchangers, ice tends to accumulate on the outer fins, and particularly in the lower parts of the exchangers at which the LNG enters. The formation of ice layers on the exchanger fins impedes the heat transfer process. Moreover, the so formed ice layers may be unevenly distributed along the tubes, which adds weight to the exchangers and may even change the center of gravity of the exchanger. Excessive ice layer formation is particularly problematic where stringent structural code requirements for wind and seismic loads need to be met.
Where ice layers have already built up to an unacceptable level that reduces the overall heat transfer, the LNG vaporization process must often be stopped and the exchangers are then placed on a standby de-icing cycle. In most cases, de-icing is done by natural draft convection, which is very time consuming. To reduce de-icing time, force draft air fans may be employed. However, such operation reduces the defrosting time only marginally as heat transfer is limited by the ice layer that acts as an insulator. The use of forced air fan is also difficult to be justified due to additional cost and energy consumption of the air circulation fans. Typically, over one-third of the ambient air vaporizers are on defrosting and the other two-third are on LNG regasification. Furthermore, performance of such known ambient air vaporizers are sensitive to changes in environmental factors such as variations in humidity and dry bulb temperature, ambient temperature fluctuations, relative humidity, wind, solar radiation, and/or surrounding structures.
Therefore, while numerous configurations and methods of ambient air vaporization of LNG are known in the art, all or almost all of them suffer from one or more disadvantages. Thus, there is still a need to provide improved configurations and methods for regasification of LNG.